Hepatic-vagal and gustatory afferent interactions in the brainstem of the rat.

In a series of combined electrophysiological and peroxidase histochemical studies, we have established that first-order hepatic and gustatory afferents project to separate regions of the solitary nucleus (NST) and do not converge upon neurons within this structure. Additionally, we have found that hepatic vagal afferent fibers travel in the left cervical vagal trunk and project to the postero-medial division of the left NST. Very small (100-200 micron diameter) iontophoretic injections of horseradish peroxidase (HRP) into physiologically identified zones of the NST reveal that the immediately subjacent parvocellular reticular formation sends efferents to and receives afferents from both the hepatic and gustatory divisions of this nucleus. Further, hepatic and gustatory-NST regions send projections throughout the parabrachial nucleus (PBN). Axons from both the hepatic and gustatory regions of the NST terminate heavily in the posterior dorsomedial part of the PBN. At anterior levels, the gustatory and visceral NST projections are distinctly separate; the medial PBN receives input from gustatory-NST, while the lateral region received input from the hepatic-NST. These results provide anatomical support for our previous findings that single neurons within the PBN can be activated by both gustatory and hepatic afferent activation. Overlapping projections from hepatic (vagal) and gustatory regions of the NST within both the immediately subjacent parvocellular reticular formation as well as the postero-medial PBN may explain in part the decerebrate rat's ability to not only discriminate gustatory stimuli, but also switch its response to a constant stimulus dependent on its nutritive state.